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    Re: On the integration of location and data
    From: George Huxtable
    Date: 2009 Nov 4, 17:44 -0000

    I wrote, in [10411] "It was the result of unfeasible claims (later
    withdrawn) made by Celestron's publicity people about its insensitivity to
    distortion of the Earth's magnetic field. And it was on the basis of those
    claims that some fanciful projections were made on Nav-list about what might
    become possible in terms of pointing-precision; claims which are now being
    repeated."
    
    To which Frank responds, in capitals-
    "WHAT claim has been repeated??"
    
    In 10400, Frank wrote- "here's a link to a message of mine in that earlier
    thread: http://fer3.com/arc/m2.aspx?i=108258)."
    That was message [8258], back on 14 May. Which went into some detail about
    achievable pointing precision in the "niche market astronomy toy" ...
    Celestron Skyscout, as follows-
    "You ... turn it on, point it at a star, and it tells you just what you're
    looking at....it can determine where you're pointing in the sky from
    anywhere on Earth at any date and time ... with an accuracy of about 0.5
    degrees. ...if there's significant magnetic or acceleration interference
    then it would have problems. ... It's not bothered by minor magnetic
    interference"
    
    That was the claim that presumably Frank is repeating, as he has linked our
    attention to it once again. Does he still maintain those words?
    
    At the time, I expressed my usual scepticism. It seemed too good to be true.
    It was. Being entirely dependent on the local magnetic field, its pointing
    accuracy could be no better than the local magnetic field was. The ensuing
    discussion flows under the threadname "learn the stars, by phone".
    
    On investigation, it turned out that Celestron had quickly relaxed that spec
    to a pointing precision of 0.5 to  "2 to 3 degrees", interpreted as 3
    degrees by a Celestron distributor.
    
    We were assured that the instrument could detect the presence of local
    magnetic material, and give a warning, before significant error occurred.
    That seemed unlikely; see recent results, below.
    
    So that implied that the pointing could be off-axis by 3 degrees, and the
    instrument could still meet its revised spec., a target width 6 degrees
    across (about 3 fingers at arm's length). Celestron claimed that their
    database included 50,000 sky objects: that target might contain 34 of them.
    
    In the absence of any magnetic perturbation, no doubt that instrument, or
    some sort of fancy-phone that works the same way, might do a reasonable job
    in pointing precision, though not, unfortunately, in a moving envirenment at
    sea or on land. With a firm footing in the open air, away from vehicles,
    steel-framed buildings, a pyrites seam, or steel-framed spectacles, it might
    be fine as long as the magnetic mapping gets updated periodically.
    Elsewhere, how can a user be sure that it's unaffected, in a world that's
    rife with iron and steel?
    
    Not by relying on the indicator that warns of the presence of magnetic
    material. An attempt, not by me, to try fooling it, showed that it was
    possible, with care,  to approach a Skyscout, from East or West, with a
    small magnet held horizontally, to within 10 cm, and thereby twist the
    indicated azimuth by all of 50 degrees, before it indicated presence of
    magnetic material!
    
    In his recent posting, Frank appears to be rowing back from magnetics, by
    accepting that the technology  "generally implies a built-in magnetic
    compass. There's a way around this in major cities if you use some
    sophisticated processing (which can occur server-side) to take the image,
    the GPS (or wifi) location data and then figure out direction based on the
    buildings seen and maybe even using shadows if it's daytime. That combined
    with the simple inertial sensors in many phones would yield orientation
    without a magnetic compass."
    
    Is Frank really serious? In what circumstances could such technology
    conceivably aid a viewer of the night sky? We've had many absurdities on
    Navlist, but this one ranks near the best.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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